1. Field of the Invention
The present invention generally relates to high bandwidth interconnections for use in networking environments such as local area networks (LAN), wide area networks (WAN) and storage area networks (SAN). More specifically, it relates to a method of correcting skew in signals resulting from different paths lengths and obstructions in multiple, parallel signal carriers.
2. Description of Related Art
Internet and electronic commerce has grown to the point where demands placed on existing computer systems are severely testing the limits of system capacities. Microprocessor and peripheral device performances have improved to keep pace with emerging business and educational needs. For example, internal clock frequencies of microprocessors have increased dramatically, from less than 100 MHz to more than 1 GHz over a span of less than ten years. Where this performance increase in inadequate, high performance systems have been designed with multiple processors and clustered architecture. It is now commonplace for data and software applications to be distributed across clustered servers and separate networks. The demands created by these growing networks and increasing speeds are straining the capabilities of existing Input/Output (I/O) architecture.
Peripheral Component Interconnect (PCI), released in 1992, is perhaps the most widely used I/O technology today. PCI is a shared bus-based I/O architecture and is commonly applied as a means of coupling a host computer bus (front side bus) to various peripheral devices in the system. Publications that describe the PCI bus include the PCI Specification, Rev. 2.2, and Power Management Specification 1.1, all published by the PCI Special Interest Group. The principles taught in these documents are well known to those of ordinary skill in the art and are hereby incorporated herein by reference.
At the time of its inception, the total raw bandwidth of 133 MBps (32 bit, 33 MHz) provided by PCI was more than sufficient to sustain the existing hardware. Today, in addition to microprocessor and peripheral advancements, other I/O architectures such as Gigabit Ethernet, Fibre Channel, and Ultra3 SCSI are outperforming the PCI bus. Front side buses, which connect computer microprocessors to memory, are approaching 1–2 GBps bandwidths. It is apparent that the conventional PCI bus architecture is not keeping pace with the improvements of the surrounding hardware. The PCI bus is quickly becoming the bottleneck in computer networks.
In an effort to meet the increasing needs for I/O interconnect performance, a special workgroup led by Compaq Computer Corporation developed PCI-X as an enhancement over PCI. The PCI-X protocol enables 64-bit, 133 MHz performance for a total raw bandwidth that exceeds 1 GBps. While this is indeed an improvement over the existing PCI standard, it is expected that the PCI-X bus architecture will only satisfy I/O performance demands for another two or three years.
In addition to the sheer bandwidth limitations of the PCI bus, the shared parallel bus architecture used in PCI creates other limitations which affect its performance. Since the PCI bus is shared, there is a constant battle for resources between processors, memory, and peripheral devices. Devices must gain control of the PCI bus before any data transfer to and from that device can occur. Furthermore, to maintain signal integrity on a shared bus, bus lengths and clock rates must be kept down. Both of these requirements are counter to the fact that microprocessor speeds are going up and more and more peripheral components are being added to today's computer systems and networks.
Today, system vendors are decreasing distances between processors, memory controllers and memory to allow for increasing clock speeds on front end buses. The resulting microprocessor-memory complex is becoming an island unto itself. At the same time, there is a trend to move the huge amounts of data used in today's business place to storage locations external to network computers and servers. This segregation between processors and data storage has necessitated a transition to external I/O solutions.
One solution to this I/O problem has been proposed by the Infiniband(SM) Trade Association. The Infiniband(SM) Trade Association is an independent industry body that is developing a channel-based, switched-network-topology interconnect standard. This standard will de-couple the I/O subsystem from the microprocessor-memory complex by using I/O engines referred to as channels. These channels implement switched, point to point serial connections rather than the shared, load and store architecture used in parallel bus PCI connections.
The Infiniband interconnect standard offers several advantages. First, it uses a differential pair of serial signal carriers, which drastically reduces conductor count. Second, it has a switched topology that permits many more nodes which can be placed farther apart than a parallel bus. Since more nodes can be added, the interconnect network becomes more scalable than the parallel bus network. Furthermore, as new devices are added, the links connecting devices will fully support additional bandwidth. This Infiniband architecture will let network managers buy network systems in pieces, linking components together using long serial cables. As demands grow, the system can grow with those needs.
The trend towards using serial interconnections as a feasible solution to external I/O solutions is further evidenced by the emergence of the IEEE 1394 bus and Universal Serial Bus (USB) standards. USB ports, which allow users to add peripherals ranging from keyboards to biometrics units, have become a common feature in desktop and portable computer systems. USB is currently capable of up to 12 MBps bandwidths, while the IEEE 1394 bus is capable of up to 400 MBps speeds. A new version of the IEEE 1394 bus (IEEE 1394b) can support bandwidth in excess of 1 GBps.
Maintaining signal integrity is extremely important to minimize bit error rates (BER). At these kinds of bandwidths and transmission speeds, a host of complications which affect signal integrity may arise in the physical layer of a network protocol. The physical layer of a network protocol involves the actual media used to transmit the digital signals. For Infiniband, the physical media may be a twisted pair copper cable, a fiber optic cable, or a copper backplane. Interconnections using copper often carry the transmitted signals on one or more pairs of conductors or traces on a printed circuit board. Each optical fiber or differential conductor pair is hereafter called a “lane”.
Where multiple lanes are used to transmit serial binary signals, examples of potential problems include the reordering of the lanes and skew. Skew results from different lane lengths or impedances. Skew must be corrected so that data that is transmitted at the same time across several lanes will arrive at the receiver at the same time. Lane reordering must be corrected so a digital signal may be reconstructed and decoded correctly at the receiver end.
Even in the simplest case involving a single differential wire pair, a potential problem exists in the routing of the differential wire pair. It is possible for wires to be crossed either inadvertently, as in a cable miswire, or intentionally, as may be necessary to minimize skew. In transmitting digital signals via a differential wire pair, one wire serves as a reference signal while the other wire transmits the binary signal. If the wire terminations are incorrect, the binary signal will be inverted.
Conventional correction and prevention of these types of problems has been implemented with the meticulous planning and design of signal paths. Differential wire pairs are typically incorporated into cables as twisted wire pairs of equal lengths. However, matched delay or matched length cabling is more expensive because of the manufacturing precision required. In backplane designs, trace lengths may vary because of board congestion, wire terminations and connector geometries. Shorter traces are often lengthened using intentional meandering when possible to correct for delay caused by other components. It is often impractical to correct crossed differential pairs because one trace passes through two vias to “cross under” the other trace. The vias introduce a substantial time delay, thereby causing data skew. Alternatively, the differential pairs are left uncorrected and the data inversion is accounted for using pin straps or boundary scan techniques. Both of these fixes require intervention by the system designer. These techniques have also been used to correct lane reversal.
The physical layer in Infiniband carries signals encoded by a digital transmission code called “8B/10B”. 8B/10B is an encoding/decoding scheme which converts an 8-bit word (i.e., a byte) at the link layer of the transport protocol to a 10-bit word that is transmitted in the physical layer of the same protocol.
The 8B/10B code is a “zero-DC” code, which provides some advantages for fiber optic and copper wire links. Transmitter level, receiver gain, and equalization are simplified and their precision is improved if the signals have a constant average power and no DC component. Simply stated, in converting an 8-bit word to a 10-bit word, the encoder selects the 10-bit representation based on the sign of the running disparity of the digital signal. Running disparity refers to a running tally of the difference between the number of 1 and 0 bits in a binary sequence. If the running disparity is negative (implying that more 0 bits have been transmitted than 1 bits), the subsequent 8B/10B word will contain more 1 bits than 0 bits to compensate for the negative running disparity. In the 8B/10B code, every 8-bit word has two 10-bit equivalent words. The 10 bit equivalent words will have five or more 1 bits for a negative running disparity and five or more 0 bits for a positive running disparity. For a more detailed description of the 8B10B code, refer to Widmer and Franaszek, “A DC-Balanced, Partitioned-Block, 8B/10B Transmission Code”, IBM J. Res. Develop., Vol. 27, No. 5, September 1983, which is hereby incorporated by reference.
The above design considerations clearly make physical layer (i.e., cables, backplanes) manufacturing a difficult venture in high clock frequency systems. Design costs and manufacturing costs are drastically increased due to the need to alleviate these types of problems. It is desirable, therefore, to provide a method of automatically correcting these types of errors with information embedded in the signals. It is further desirable to develop a method of automatically detecting and correcting signal skew to coordinate synchronous arrival of signals across multi-lane serial links. This method of correction may advantageously allow for less stringent design requirements and could decrease design and manufacturing costs for high bandwidth data links.